Small cells are thought by many to be a promising component of future 5th Generation (5G) wireless systems. However, the potential of small cells can only be realized if installation costs can be controlled.
Two essential requirements for a small cell installation are electrical power and backhaul, which when provided by wired connections are often costly. A wired connection for electrical power may be eliminated by charging storage batteries using power derived from, for example, a small wind turbine and/or solar panel, while minimizing internal power consumption. The wired backhaul connection may be eliminated by using a wireless backhaul.
A relay cell has been proposed to reduce internal power consumption at a small cell. On the downlink, the relay cell receives a complex-valued analog baseband signal on a carrier frequency from a backhaul link and retransmits the baseband signal at an access carrier frequency that accommodates one or more users attached to the relay cell. On the uplink, the relay cell receives combined transmitted signals from one or more users on an access link, and retransmits the combined baseband signals on the backhaul uplink. This type of relay cell is designed for relatively low-power operation.
Conventional wireless backhaul connections may be provided either by radio frequencies between about 0.8 to 6.0 GHz (e.g., Wi-Fi) or by millimeter-wave technology. While both of these technologies may be practical under certain operating conditions, each also has limitations. For radio frequencies between about 0.8 to 6.0 GHz, under non line-of-sight conditions, the required radiated power for the uplink increases approximately 10 decibels (dB) for every doubling of range (with an assumed loss-exponent of about 3.5). This limits the operating range of backhaul connections using these frequency bands, and hence the flexibility of small cell deployment using this technology. Millimeter-wave backhaul connections take advantage of physically small high-gain dish antennas to compensate for range-induced attenuation loss. However, millimeter-wave links are generally limited to line-of-sight propagation and require relatively tight alignment of antennas, which again limits the flexibility of deployment. Moreover, millimeter-wave links are not considered “all-weather”.